The present disclosure generally relates to induction heating, and more particularly to an induction heating apparatus capable of detecting a resonant frequency of a resonant power inverter for the induction heating apparatus.
Induction cook-tops heat conductive cooking utensils by magnetic induction. An induction cook-top applies radio frequency current to a heating coil to generate a strong radio frequency magnetic field on the heating coil. When a conductive object or vessel, such as a pan, is placed over the heating coil, the magnetic field coupling from the heating coil generates eddy currents on the vessel. This causes the vessel to heat.
In order to properly drive the induction cook-top or heating system, it is important to have an accurate assessment of the resonant frequency of the resonant power inverter being used to drive the induction cooktop. Operating the resonant power inverter at the proper frequency such as at, or slightly above resonance, can be advantageous for a number of reasons. Some of these reasons include, for example, achieving maximum power transfer between the induction heating coil and the object or vessel on the induction heating coil, and maintaining safe working and operating conditions. Operating the induction system at a sub-resonant frequency can result in damage to the induction heating system due to limitations of a half bridge resonant inverter power supply.
The resonant frequency of the resonant power inverter can also provide information as to the load conditions of the induction heating coil. This information can include, for example, the size and type of object that is placed on the induction cook-top. One example of a system for detecting an object on an induction cooktop and correspondingly controlling power to the induction heating coil is disclosed in U.S. patent application Ser. No. 13/154,190 entitled “Induction Cooktop Pan Sensing”, filed on Jun. 6, 2011 and assigned to the assignee of the instant application, the disclosure of which is incorporated herein by reference in its entirety.
There are multiple methods of object or vessel detection on an induction cook-top. Some of these include mechanical switching, phase detection, optical sensing and harmonic distortion sensing. In some systems, these detection methods typically use a current transformer to detect the resonant voltage. When the system is operating at resonance, optimal power transfer between the induction heating coil and the object on the induction heating coil will occur. However, a current transformer will always provide a clean sine wave of power output to the induction heating coil, whether the system is operating in resonance or non-resonance. The sinusoidal nature of the output signal produced by the current-transformer is not dependent upon resonance and there will be little to no distortion due to switching. Also, current-transformer packages tend to have large package sizes and footprints, and can be expensive.
Accordingly, it would be desirable to provide a system that addresses at least some of the problems identified above.